1. Field of the Invention
The invention relates to an organic polymer siloxane compound containing sulfonic acid groups and a fuel cell including the same, and more particularly, to an organic polymer siloxane compound containing sulfonic acid groups which, by reducing swelling, has superior dimensional stability and ionic conductivity without affecting the amount of methanol crossover, and a fuel cell including the same.
2. Description of the Related Art
A fuel cell is an electrochemical device which directly transforms chemical energy of hydrogen and oxygen, which are contained in hydrocarbon materials such as methanol, ethanol, and natural gas, into electric energy. The energy transformation process of fuel cells is very efficient and environmentally-friendly, thereby drawing attention for the past several decades, and the development of various types of fuel cells has been attempted.
Fuel cells can be classified into Phosphoric Acid Fuel Cells (PAFC), Molten Carbonate Fuel Cells (MCFC), Solid Oxide Full Cells (SOFC), Polymer Electrolyte Membrane Fuel Cells (PEMFC), and Alkaline Full Cells (AFC) according to the type of electrolyte used. All fuel cells operate on the same principle, but the type of fuel, operating temperature, catalyst, and electrolyte used are different. In particular, PEMFCs can be used in small-sized stationary power generation equipment or transportation systems due to low operating temperature, high output density, rapid start-up, and power response to variations of required output.
The core part of a PEMFC is a Membrane and Electrode Assembly (MEA). A MEA includes a polymer electrolyte membrane and two electrodes which are generally attached to both sides of the polymer electrolyte membrane and independently act as a cathode and an anode.
The polymer electrolyte membrane acts as a separator blocking direct contact between an oxidizing agent and a reducing agent, and electrically insulates the two electrodes while conducting protons. Accordingly, a good polymer electrolyte membrane has high proton conductivity, good electrical insulation, low reactant permeability, excellent thermal, chemical, and mechanical stability under normal fuel cell conditions, and a low price.
In order to meet these requirements, various types of polymer electrolyte membranes have been developed, and, in particular, high fluorinated polysulfonic acid membranes have been developed due to their excellent durability and performance and one of these, a NAFION® membrane is a standard. However, the NAFION membrane needs to be sufficiently moisturized and used at 80° C. or less to prevent moisture loss.
Moreover, in a Direct Methanol Fuel Cell (DMFC), an aqueous methanol solution is supplied as a fuel to the anode and a portion of non-reactive aqueous methanol solution is permeated to the polymer electrolyte membrane. The non-reactive aqueous methanol solution that permeates to the polymer electrolyte membrane causes a swelling phenomenon in the polymer electrolyte membrane, and diffuses a cathode catalyst layer. Such a phenomenon, referred to as “methanol crossover,” causes the direct oxidization of methanol at the cathode where an electrochemical reduction of hydrogen ions and oxygen occurs, and thus the methanol crossover results in a drop in electric potential, causing a decline in the performance of the DMFC.
Since conventional polymer electrolyte membranes do not have both excellent ionic conductivity and low methanol crossover, improvements in polymer electrolyte membranes are required.